1. Field of the Invention
The present invention relates to a coil of an electric machine including a generator and a motor, and more particularly relates to a field coil of an alternator for a vehicle and a manufacturing method thereof.
2. Description of the Related Art
U.S. Pat. No. 5,174,013 (which corresponds to JP-A-63-190310) and JP-A-2-243468, filed by the same assignee, propose a rotor of an alternator which has a field coil or rotor coil of magnet wire having a polygonal (rectangular or hexagonal) cross-section formed by a forming roller mechanism. Such magnet wire is, as indicated by a reference numeral 110 in FIG. 14, wound along a spiral groove 101 formed as a guide on a cylindrical surface of a bobbin 100 to form a first layer (bottom layer) and subsequently wound along a spiral groove formed between adjacent magnet wires of the first layer to form a second layer. The magnet wire of the third layer is formed on the second layer, and the same process is repeated to form the rotor coil having a designated number of turns. Thus, the space factor of the rotor coil is increased.
After the magnet wire 110 is wound around the bobbin 100, pole cores 120 are inserted into a center hole of the bobbin 100 from opposite sides thereof. At that time the magnet wire 110 in upper layers is pressed by inclined inner surfaces of claw portions 121 of the pole cores 120 as shown in FIG. 14. Because the layers of the polygonal magnet wire are held tighter in place by itself than the common round magnet wire, the magnet wire can not avoid the pressure from the claw portions.
As a result, coating 111 (shown in FIG. 15) of a corner of the magnet wire 110b in the upper layer, which crosses a corner of the magnet wire 110a in the lower layer, is subject to damage and conductor member (or copper core) 112 may become exposed since insulating coating 111 of the corner is generally thinner than that of other portions. In addition, since the coil 110 has the polygonal cross-section, the corner of the magnet wire 110 in the upper most layer may dig into an insulating flange 102 of the bobbin 100 (made of resinous material) disposed between the claw portions 121 and the rotor coil.
As a result, the conductor member 112 may come into contact with the pole core 120 as shown in FIG. 14, causing insulation trouble and a decrease of the output power of the generator.